About 100,000 children fell into relative poverty in 2015-15, taking the overall figure to 4 million.
Photograph: Christopher Furlong/Getty Images

The upward trend in child poverty in the UK has continued for the third year running, with the percentage of children classed as poor at its highest level since the start of the decade, latest official figures show.

About 100,000 children fell into relative poverty in 2015-16, a year on year increase of one percentage point, according to household data published by the government on Thursday. About 4 million, or around 30%, are now classed as poor.

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Campbell Robb, the chief executive of the Joseph Rowntree Foundation, said: “These troubling figures are warning signs we could be at the beginning of a sharp rise in poverty, with forecasts suggesting child poverty could rise further by 2021.”

The data showed that nearly half of single-parent children are poor, with a noticeable surge in poverty over the past year among children of lone parents who work full-time. About 67% of the UK’s poor children are from working families.

“The prime minister spoke about injustice on entering Downing Street, but there is no greater burning injustice than children being forced into poverty as a result of government policy,” said the Child Poverty Action Group’s chief executive, Alison Garnham.

The government said the wider household data showed the UK economy was strong. Household incomes have risen, income inequality is lower than in 2010, and the number of children in workless households has fallen.

The work and pensions secretary, Damian Green, said: “I’m committed to tackling disadvantage and these figures confirm that work is the best route out of poverty. Working parents help the whole family because of the dignity and security that comes from having a job.”

The Institute for Fiscal Studies said income for working-age adults was no higher than eight years ago. Inequality and poverty remain slightly lower than before the financial crisis.

Rosie Ferguson, the chief executive of Gingerbread, the charity for single parents, said: “Child poverty is being allowed to fester rather than being tackled head on. That nearly half of all children in a single-parent family are now in poverty is a shocking statistic. ”

Justin Watson, the head of Oxfam’s UK programme, said:“There are now more people in poverty in the UK than there have been for almost 20 years and a million more than at the beginning of the decade.”